1. Field of Invention
The invention relates to polymer particles, particularly latex polymer particles, that may be used to form toner. The resulting toners can be selected for known electrophotographic imaging and printing processes, including digital color processes, and are especially useful for imaging processes, specifically xerographic processes, which usually require high toner transfer efficiency, such as those having a compact machine design without a cleaner or those that are designed to provide high quality colored images with excellent image resolution and signal-to-noise ratio, and image uniformity, and for imaging systems wherein excellent glossy images are generated.
2. Description of Related Art
Numerous processes are known for the preparation of toners. For example, in conventional processes, a resin is melt kneaded or extruded with a colorant, particularly a pigment, and the product thereof is micronized and pulverized to provide toner particles. The toner particles formed by this process generally have an average volume particle diameter of from about 7 microns to about 20 microns and a broad geometric size distribution of from about 1.4 to about 1.7. As a result, it is usually necessary to subject the aforementioned toner particles to a classification procedure such that a geometric size distribution of from about 1.2 to about 1.4 is attained.
There are also several so-called chemical processes for making toner, among them is the aggregation/coalescence process for making toner particles. In this process, narrow particle size distribution can be achieved without classification. In this process, the resin is prepared as a water based dispersion of sub-micron sized polymeric particles (polymeric latex), which are then aggregated with pigment particles of sub-micron size to the desired toner size and are then coalesced to produce pigmented toner particles.
U.S. Pat. No. 5,853,943, which is herein incorporated in its entirety by reference, is directed to a process for preparing a latex polymer by emulsion polymerization. In this process, the latex polymer is formed by first forming a seed polymer. To form toner from the latex polymer, U.S. Pat. No. 5,853,943 discloses blending the latex with a colorant dispersion; heating the resulting mixture at a temperature below or equal to the Tg of the polymer in the latex to form toner sized aggregates; and heating the aggregates at a temperature at or above the Tg of the polymer to coalesce or fuse the components of the aggregates.
A wide variety of polymer types are used in forming the polymer particles of toner. The polymers include both homopolymeric and copolymeric compositions, such as styrene-butadiene-acrylic acid copolymers, styrene-butyl acrylate-acrylic acid copolymers and acrylic homopolymers.
By selecting various homopolymers and copolymers, toners can be generated that possess specific chemical, mechanical and/or triboelectrical properties. In particular, toners with a low minimum fixing temperature (MFT) are desired to, for example, reduce the energy requirements of the printers and copiers, and to further extend the lifetime of the fuser rolls. However, reducing the MFT of the toner may cause other properties of the toner to be diminished.
For example, as described in U.S. Pat. No. 5,928,830, in pictorial or process color applications, the gloss provided by the toner resin is important to the attainment of high image quality. Unfortunately, a latex that has the desired fix properties may not yield acceptable gloss properties. In particular, if a latex resin has a low molecular weight, that is, for example, a weight average molecular weight (Mw) of about 5,000 to about 30,000, as determined by Gel Permeation Chromatography (GPC), on some fusing devices, the latex resin may result in a developed toner image with an excellent gloss, of, for example, greater than 50 gloss units, such as 70-90, for high quality color applications. However, the toner may have poor fix, that is the MFT may be higher than about 190.degree. C. to about 220.degree. C. for the resulting toner. In contrast, if a latex has a high molecular weight, such as an Mw of about 35,000 to about 80,000, then the latex could result in poor gloss and excellent fix on the same fusing devices.
In addition, as described in U.S. Pat. No. 5,604,076, for certain xerographic properties, such as low minimum fixing temperature, non-vinyl offset characteristics and high gloss properties, polyester resins may be advantageous in comparison to styrene based resins. In contrast, styrene based tone resins may be advantages in comparison to polyester resin for certain properties such as low relative humidity sensitivity, high blocking temperatures and lower unit manufacturing cost.
U.S. Pat. No. 5,496,676, which is herein incorporated in its entirety by reference, suggests blending various latexes to optimize various toner properties. However, it is often difficult to blend various latexes based on differences between them that provide for limited compatibility. In particular, the latexes may be composed of monomers of different classes and/or species. In addition, the different latexes may have different particle surface properties, glass transition temperature and molecular weight. This in turn may cause the resin to phase separate when heated together, providing for domains that are rich in each resin, and thus form separately aggregated particles.
In addition, it is known in the art to copolymerize various monomers together. However, this is not always satisfactory. In particular, toner gloss and fix are predominantly affected by the molecular weight of the latex in contrasting ways. Therefore, the mere copolymerization of various monomers may not allow for the adjustment of the molecular weight, which is suitable for both toner fix and gloss applications.
U.S. Pat. No. 5,928,830, which is herein incorporated in its entirety by reference, is directed to a process for forming latex particles for use in toner in which a core polymer is encapsulated by a shell polymer. By using a core-shell latex, one can select the optimum properties for each of the core and shell resins, which otherwise may not readily be obtainable by a single latex. In embodiments of the invention described therein, the core polymer has a glass transition temperature (Tg) of about 20.degree. C. to about 50.degree. C. and a weight average molecular weight (Mw) of about 5,000 to about 30,000 and the shell polymer has a Tg of about 50.degree. C. to about 70.degree. C. and a Mw of 30,000 or higher.
U.S. Pat. No. 5,604,076, which is herein incorporated in its entirety by reference, is directed to a process for forming toner in which the latex particles comprise a polyester core encapsulated within a styrene based resin shell. In the core-shell latex, the surface characteristics of the toner are directed by the encapsulant component, such as polystyrene-acrylic acid. These surface characteristics include blocking temperature, triboelectric characteristics and RH sensitivity provided by the acid residual. In addition, the polyester core provides for low MFT, high gloss properties and excellent nonvinyl offset performance.
U.S. Pat. No. 4,717,750, which is herein incorporated in its entirety by reference, is directed to a process for preparing a structural reinforced latex particle having improved tensile and elongation properties by emulsion polymerization comprising a three-stage monomer addition. In the process, the second monomer feed forms a polymer having a higher Tg than the first and third monomer feeds.